A thick film circuit is being adapted for use in many types of electronic equipment such as broadcasting radio receivers, television receivers, video tape recorders, communication equipment, and measuring equipment.
A thick film circuit is generally formed using the screen printing method. More specifically, a desired pattern is formed on a substrate made of alumina ceramic using a conductor paste, resistor paste or the like, followed by drying and heating so as to manufacture a thick film circuit. However, the screen printing method uses a mask pattern, with the results that longer time is required for the exchange of the mask pattern, and cleaning of the mask pattern and squeegee, and thus it is unsuitable for small lot production and frequent changes of pattern.
On the other hand, a method, which involves drawing on a moving substrate using a paste directly extruded from a nozzle without employing the mask pattern, is reported by "1978 International Microelectronics Symposium of ISHM, Minneapolis, MN, Sept. 17, 1978" and is introduced by a journal "Electronic Packaging and Production (January, 1981), Pages 98-109".
In operation, a drawing nozzle having a discharge hole for discharging a paste is kept at a certain distance from a substrate and is relatively moved with respect to the substrate to apply the paste withdrawn on a predetermined pattern of a thick film circuit. This drawing method does not require the mast pattern because the thick film circuit is drawn by means of NC control, thereby allowing quick changes in the kind of pattern to instantly modify the circuit.
However, one problem with such a drawing method is that the viscosity of the paste is partially increased due to evaporation of solvent from the paste at the discharge hole during long non-drawing times, that is, the paste is dried, and the increase of the viscosity causes the occurrence of clogging. Therefore, it is required to frequently clean the discharge hole and thus difficulty is encountered to increase workability and productivity.